Field of the Invention
The present invention is concerned with tuning screws, particularly for use in microwave tuning devices where it is required to enable a tuning plunger or slug to be selectively inserted into a microwave cavity to a variable extent for correspondingly varying the reactance of the cavity.
A known tuning device (see U.S. Pat. No. Re. 30,406) comprises an externally screw threaded, electrically conductive rod which is received within an internally screw threaded electrically conductive sleeve. The threaded portion of the rod is formed with a pair of parallel, flat bottomed slots of equal depth which extend radially inwardly from diametrically opposite sides of the rod. The depth of these slots is substantially greater than the radius of the rod. The provision of the slots provides the rod with a certain amount of axial resilience which, when the rod is inserted in the sleeve in a compressed state so as to resiliently misalign the threads on the rod and sleeve, the resulting permanent deformation of the rod in the region of the slots causes an axially directed gripping force between the mating threads which serves to resist unintentional rotation of the rod.
One problem with this known device is that the torque which resists unintentional rotation of the rod within the sleeve is generated by the co-operating metal screw threads which inter-engage with a degree of mutual interference. Such an arrangement is inherently liable to generate loose metallic particles (swarf) in the joint. Since this inter-engagement of the screw threads also provides the necessary electrical connection between the rod and the sleeve, the presence of such swarf particles in the joint can cause electrical noise and other spurious effects in the waveguide system.
It is an object of the present invention to provide a tuning device in which the latter problem is mitigated.
In accordance with the present invention, there is provided a microwave tuning device having a cylindrical metal bush having an internal screw-thread, a metal, rod-like plunger received within the bore in the cylindrical metal bush so as to be axially displaceable therein, a bush of resilient plastics material disposed coaxially about a portion of the plunger adjacent one and thereof, the plastics bush being inserted into the internal screw-thread of the cylindrical metal bush thereby form the corresponding mating screw-thread on the plastics bush, and a plurality of fingers disposed on one end of the cylindrical metal bush, the fingers resiliently engaging the end opposite the one of the plunger to provide resistance to unintentional rotation of the plunger in the metal bush and electrical interconnection therebetween.
Preferably, the plastics bush is formed of P.T.F.E. and is fixed rigidly to the plunger by means of a plurality of splines and grooves formed on the outer periphery of the plunger.